1. Field of the Invention
The present invention relates to a color image forming apparatus and color image forming method and, more particularly, to a tandem type electrophotographic color image forming apparatus having independent image forming units for respective color components, and a color image forming method.
2. Description of the Related Art
As a kind of color image forming apparatus such as a printer or copying machine, there is known a tandem type color image forming apparatus which comprises electrophotographic image forming units equal in number to color components and sequentially transfers toner images of respective color components onto a print medium by the image forming units. The image forming unit of each color includes a developing unit and photosensitive drum. It is known that the tandem type color image forming apparatus has a plurality of factors which cause a positional error (to be referred to as a registration error) between images of respective color components.
These factors include the unevenness and attaching positional error of the lens of a deflecting scanning unit including the optical system of a polygon mirror, fθ lens, and the like, and the mounting positional error of the deflecting scanning unit to the image forming apparatus main body. Owing to these positional errors, the scan line does not become a straight line parallel to the rotating shaft of the photosensitive drum, and inclines or skews. If the degree of inclination or skew of the scan line (to be referred to as the profile or shape of the scan line hereinafter) is different between colors, a registration error occurs.
The profile has different characteristics for respective image forming apparatuses, that is, printing engines, and for deflecting scanning units of respective colors. FIGS. 6A to 6D show examples of the profile. In FIGS. 6A to 6D, the abscissa axis represents a position in the main scanning direction in the image forming apparatus. A line 600 expressed as a straight line in the main scanning direction represents the characteristic (profile) of an ideal scan line free from a skew. Curves 601, 602, 603, and 604 represent the profiles of respective colors, and show examples of the profiles of scan lines for cyan (to be referred to as C hereafter), magenta (to be referred to as M hereafter), yellow (to be referred to as Y hereafter), and black (to be referred to as K hereafter), respectively. The ordinate axis represents a shift amount in the sub-scanning direction from an ideal characteristic. As is apparent from FIGS. 6A to 6D, the curve of the profile is different between colors. When electrostatic latent images are formed on the photosensitive drums of image forming units corresponding to the respective colors, the profile difference appears as the registration error between image data of the respective colors.
As a measure against the registration error, Japanese Patent Laid-Open No. 2002-116394 discloses a method of measuring the degree of skew of a scan line using an optical sensor in the process of assembling a deflecting scanning device, mechanically rotating the lens to adjust the skew of the scan line, and fixing the lens with an adhesive.
Japanese Patent Laid-Open No. 2003-241131 discloses a method of measuring the inclination of a scan line using an optical sensor in the process of mounting a deflecting scanning device into a color image forming apparatus main body, mechanically tilting the deflecting scanning device to adjust the inclination of the scan line, and then mounting the deflecting scanning device into the color image forming apparatus main body.
Japanese Patent Laid-Open No. 2004-170755 discloses a method of measuring the inclination and skew of a scan line using an optical sensor, correcting bitmap image data to cancel them, and forming the corrected image. That is, a shift of an actual scan line from an ideal scan line which is a straight line parallel on the surface of the photosensitive drum to the rotating shaft of the photosensitive drum is canceled by shifting image data by the same amount in an opposite direction. This method corrects image data, and thus does not require a mechanical adjustment member or adjustment step in assembly. This method can downsize a color image forming apparatus, and deal with a registration error at a lower cost than those by methods disclosed in Japanese Patent Laid-Open Nos. 2002-116394 and 2003-241131. The electrical registration error correction is divided into correction of one pixel and that of less than one pixel. In correction of one pixel, pixels are shifted (offset) one by one in the sub-scanning direction in accordance with the inclination and skew correction amounts, as shown in FIGS. 15A to 15C. In the following description, a position where the pixel is offset will be called a scan line changing point, and the process to offset a pixel will be called a scan line changing process. In FIGS. 15A to 15C, P1 to P5 are scan line changing points.
In FIG. 15A, a profile 1501 of a scan line is corrected. The profile 1501 may also be expressed by an array of the coordinate values of pixels on a scan line, but in FIG. 15A, is expressed by approximate straight lines divided for respective areas. The scan line changing point is a position in the main scanning direction where the profile is scanned in the main scanning direction and shifts by one pixel in the sub-scanning direction. In FIG. 15A, P1 to P5 are scan line changing points. At a scan line changing point serving as a boundary, dots after the scan line changing point are shifted by one line in a direction opposite to the shift of the profile in the sub-scanning direction. This process is executed by paying attention to each line. FIG. 15B shows an example of image data shifted in the sub-scanning direction at each scan line changing point. In FIG. 15B, each hatched portion 1511 is one line before the scan line changing process, that is, one line in original image data. As a result of the scan line changing process, each line shifts in a direction in which the shift of the profile in the sub-scanning direction is canceled. FIG. 15C shows an example of image data obtained in this manner. Each hatched portion is one line before correction. In image formation, corrected image data is formed for each line. For example, normal image formation proceeds in the order of a line 1521, line 1522, . . . . After image formation, a hatched portion which forms one line in image data before correction is formed on an ideal scan line which should be originally formed. However, the scan line changing process is done for each pixel, so a shift of less than one pixel still remains in the sub-scanning direction.
A shift of less than one pixel that cannot be completely corrected by the scan line changing process is corrected by adjusting the tone value of bitmap image data by preceding and succeeding pixels in the sub-scanning direction. More specifically, when the characteristic of the profile represents an upward inclination in the scanning direction, bitmap image data before tone correction is corrected to a pixel array inclined in a direction (downward in this example) opposite to the inclination of the profile. In order to make image data close to ideal image data after correction, tone correction is executed near a scan line changing point to smooth a step at the scan line changing point. The smoothing can be achieved using the width and intensity of a laser pulse. Tone correction performed for smoothing after the scan line changing process will be called an interpolation process.
Depending on the properties of an image, there are image data which preferably undergoes the interpolation process, and image data whose image quality is degraded by the interpolation process. For example, a repetitive pattern (to be referred to as a pattern image) with the same design, a character, a thin line, and the like which can be rendered by office document creation software can be smoothed by the interpolation process, improving the visibility of information. To the contrary, if the interpolation process is performed near a scan line changing point for a continuous tone image having undergone a screen process, the density becomes uneven only near the scan line changing point, degrading the image quality. This is because, when a line growth screen is used, the interpolation process changes the thickness of a line of a screen at a scan line changing point, and the density macroscopically seems to change. If the interpolation process is done for an add-on image such as a copy forgery-inhibited pattern, the effect of the add-on image may be lost, so the interpolation process is not suitable.
Whether or not to apply the interpolation process needs to be determined in accordance with the attribute of target image data. For this purpose, there is proposed a method using continuous tone image determining units 1119 and 1121 and pattern image determining units 1120 and 1122 for respective color planes, as shown in FIGS. 11A and 11B. According to this method, decoders 1106 and 1115 combine the determination results of these units, finally deriving the interpolation determination result. The continuous tone image determining units 1119 and 1121 can determine an image for which interpolation is set off (no interpolation is executed). The pattern image determining units 1120 and 1122 can determine an image for which interpolation is set on (interpolation is executed).
When determining whether to apply the interpolation determination process, the determination result may become different between color planes. Assume that a pattern image which is formed in cyan (C), magenta (M), yellow (Y), and black (K) and matches a cyan screen pattern is input. Whether the input image is a continuous tone image is determined based on the fact that image data after the screen process contains a screen pattern. Since this pattern image has a pattern matching the cyan screen pattern, it is determined that this pattern image is a continuous tone image. At the same time, it is also determined that this input image is a pattern image having a pattern matching a predetermined pattern. It is desirable not to perform the interpolation process for a continuous tone image, and perform the interpolation process for a pattern image. If these determination results are derived, the determination result of whether or not to execute the interpolation process for this image data becomes ambiguous. Since the screen angle is generally different between colors, the input image does not match the color screen patterns of the remaining color planes, and it is determined that the input image is not a continuous tone image. Even with these remaining color planes, it is determined that the input image is a pattern image, and thus a definite determination result that interpolation is set ON can be obtained. Hence, the result of interpolation determination (called an interpolation determination result) becomes different between the cyan plane and the remaining color planes.
The pattern image should be formed with the color planes of color components used as long as it is reproduced in a combination color of color components. The interpolation determination result for a pattern image should be the same between all color planes. However, when the interpolation determination result of a given color plane becomes different from those of the remaining color planes, like the above-described example, the given color plane which forms one pattern undergoes the interpolation process, and the remaining color planes do not undergo it. A small difference between color planes may appear as an abnormal image such as a stripe or color moiré in the pattern image formed from a composition of the color planes.